Technical grade molybdenum trioxide usually contains from about 50 to about 200 weight parts per million tungsten. Because the chemistries of tungsten and molybdenum are similar, tungsten is not separated from molybdenum in conventional purification of technical grade molybdenum trioxide, such as by the process of U.S. Pat. No. 4,525,331. This is a problem because low tungsten levels, that is, &lt;150 weight ppm W is required in high purity Mo powder. Many methods are found in the literature but they are expensive or impractical.
Therefore, a method to remove tungsten from molybdenum is desirable.
A publication entitled "Use of Hydrated Oxides of Multivalent Metals For Effective Removal of Tungsten From Molybdenum Compounds", by M. I. Semenov et al published in the Journal of Applied Chem. USSR (Zh Prikl Khim. (Leningrad), 1984, 57(7), 1501-6 relates to separation of tungsten from molybdenum by adding hydrated oxides of multivalent metals to a molybdate solution containing tungsten to cause sorption of the tungsten. Their procedure is different from that of the present invention in that they do not form the hydrated tin oxide in situ or use freshly prepared hydrated tin oxide, nor do they separate tungsten from molybdenum during the ammonium hydroxide digestion of impure molybdenum trioxide. Moreover, the procedure as carried out according to this publication takes as long as seven days. With the process of the present invention the time is much shorter thereby allowing the process of the present invention to be commercially feasible.
A patent entitled "Ammonium Molybdate of High Purity" by Papageorgios, Panajotis, Plonka, Marian, Walczak, Wladylsaw, (Przedsiebiorstwo Przemyslowo-Handlowe "Polskie Odczynniki Chemiczne") Pol. 54,639 (Cl. C 01 g), 20 Jan. 1968, Appl. 28 Mar. 1966; 2 pp, relates to obtaining spectrally pure ammonium molybdate by absorption on a freshly prepared suspension of products of hydrolysis of tin salts. However, preliminary purification is recommended and methyl alcohol is used to precipitate ammonium molybdate.